I love Cho and have been playing with him for a long while now. I can tell you right now, this wonderful champion can be played in many different ways, based on the runes you have available and your preferences.
Regarding 3v3 maps, you can play him more aggressive than on 5s. Basically, you will have two choices:
a) go for magic damage (AP) and magic pen, along with cooldown reduction,
b) go for a hybrid AP/Att speed, along with cooldown reduction.
This, of course, will be regarding runes and masteries.
As for the items, it will ALWAYS depend on the enemy composition. Of course, there are some staple items that your Cho will probably need.
One thing Ive learned, though, is to ALWAYS try to get enough cooldown reduction to spam your ult. With full 40% cdr, Cho's ult can be used every 36 SECONDS! That means you might be able to use it twice in a teamfight, if it drags long enough!!
So back to your question: Its up to you to decide if you want more AP or hybrid. Personally, I prefer hybrid for jungling. Nashors tooth will give you the much needed mana regen, slight ap, and att speed needed to jungle. If you take magic pen and/or armor pen runes/masteries, you'll jungle fast and easy, even without leveling your E too much.
What I really like to do is get about 20 armor pen and 20 magic pen, that way when I fight an enemy 1v1, I usually shred him (since autoattacks are phys, but the E passive has a huge magic).
Finally, you'll probably need a defensive item. If enemy has mixed damage, RoA is always a good option. Otherwise, abyssal scepter will provide magic resist against AP enemies (as well as a neat passive for your spells and E). Frozen heart can be useful, but after testing it, I think warden's is much more useful in 3v3. Wardens mail is cheaper, and will lower enemies att AND move speed if they hit you, while glacial shroud will provide some cdr, but the att-speed debuff only comes when you get FH. Additionally, when you get the randuins, you can use it to escape (or close in) by slowing your enemies!
Enjoy your Cho'Gath